Lithium secondary batteries, which are lightweight and have high energy density, are mainly commercialized as the power source for portable devices. Also, lithium secondary batteries are currently receiving attention as large-sized, high-output power sources (e.g., power sources for automobiles), and their active development is underway.
In lithium secondary batteries, an insulating layer is provided between the positive electrode and the negative electrode. The insulating layer has the function of electrically insulating the positive and negative electrode plates from each other while retaining an electrolyte. A resin insulating layer is widely used, but such a resin insulating layer easily shrinks. Thus, when a lithium secondary battery is stored in a very high temperature environment for an extended period of time, an internal short-circuit tends to occur due to a physical or direct contact between the positive electrode and the negative electrode. The prevention of an internal short-circuit is becoming an increasingly important technical problem to be solved particularly in view of the recent trend of the insulating layer becoming increasingly thinner as the capacity of lithium secondary batteries is becoming increasingly higher. Once an internal short-circuit occurs, the short-circuit further expands due to Joule's heat generated by the short-circuit current. In some cases, the battery may overheat.
It is also very important to assure safety when batteries are internally short-circuited. Thus, techniques to enhance the safety of batteries under internal short-circuit conditions are being extensively developed. For example, Japanese Laid-Open Patent Publication No. 2004-247064 proposes a technique in which insulating tape is affixed to the exposed part of a current-collecting terminal of a positive electrode or negative electrode to prevent an internal short-circuit between the current-collecting terminals. Also, Japanese Laid-Open Patent Publication No. Hei 10-106530 proposes a technique in which an ion-permeable insulating layer composed of ceramic particles and a binder is printed on an electrode plate.
Further, to assure safety under an internal short-circuit condition, it is also very important to accurately evaluate the safety of a battery under an internal short-circuit condition. Battery evaluation tests for evaluating exothermic behavior under an internal short-circuit condition as a safety item of batteries such as lithium ion secondary batteries are defined, for example, by UL standards for lithium batteries (UL1642) and Standards of Battery Association of Japan (SBA G1101-1997 lithium secondary battery safety evaluation standard guidelines). These test methods are employed, for example, in Japanese Laid-Open Patent Publication No. Hei 11-102729 to evaluate battery safety.
Conventional evaluation tests include, for example, nail penetration tests and crush tests. A nail penetration test is an internal short-circuit test which is conducted by causing a nail to penetrate through a battery from its side face or sticking a nail thereinto. When a nail is stuck, a short-circuit occurs between the positive electrode, the negative electrode, and the nail inside the battery, so that a short-circuit current flows through the short-circuit while generating Joule's heat. Safety is evaluated by observing changes in battery temperature, battery voltage, etc., based on these phenomena. Also, a crush test is an internal short-circuit test which is performed by physically deforming a battery by using a round bar, square bar, flat plate, etc. In this manner, an internal short-circuit is caused between the positive electrode and the negative electrode, and safety is evaluated by observing changes in battery temperature, battery voltage, etc.
However, these conventional battery evaluation methods fail to accurately evaluate safety under internal short-circuit conditions. Also, in considering the uses of a battery, it is necessary to understand which level of safety performance the battery has in the event of an internal short-circuit, such as “generates no heat” or “generates a little heat”. However, since conventional methods fail to accurately evaluate safety under internal short-circuit conditions, the level of safety is not specified.
First, with respect to safety under internal short-circuit conditions, the present inventors have found that the safety of a battery in the event of an internal short-circuit changes greatly depending on the location of the short-circuit inside the battery (e.g., distance from battery surface, exposed part of current-collecting terminal, electrode active material layer), the shape of the battery, etc. For example, the safety in the event of a short-circuit near the surface of a battery is apparently high, compared with that in the event of a short-circuit at an inner part of the battery, because of the influence of heat radiation. Also, when short-circuits occur simultaneously at a location where low-resistant members such as electrode current-collecting terminals are opposed to each other and a location where relatively high-resistant members such as electrode active material layers are opposed to each other, most of the short-circuit current resulting from the short-circuits flows through the location where the low-resistant current-collecting terminals are opposed to each other. Hence, most of the Joule's heat is also generated at the location where the current-collecting terminals are opposed to each other, not the location where the active material layers with poor thermal stability are opposed to each other. As a result, the safety under internal short-circuit conditions is apparently high.
That is, depending on where a short-circuit occurs, even a battery which can be more dangerous may be improperly evaluated as safe if the evaluation method is not appropriate. In order to accurately evaluate the safety of a battery under an internal short-circuit condition, it is very important to cause an internal short-circuit at a desired location that is not in an area leading to improper evaluation of being apparently safe, in view of the battery constitution such as shape and internal structure.
In the case of nail penetration tests, the locations of short-circuits are limited to an outer part of a battery, particularly an outermost part. The evaluation results are therefore greatly affected by the constitution of the outer part of the battery. For example, the amount of heat W (W) generated by a short-circuit in a nail penetration test is as follows:W=V2×R1/(R1+R2)2 where V represents the battery voltage (V), R1 represents the resistance (Ω) of the short-circuit, and R2 represents the internal resistance (Ω) of the battery. Thus, as the resistance of the short-circuit increases, the amount of heat generated by the short-circuit becomes maximum, and as the resistance of the short-circuit decreases, the amount of heat generation decreases. That is, in a nail penetration test, when the outermost part where a short-circuit can occur is provided with a low resistant part, such as an exposed part of a current-collecting terminal on which there is no active material layer, the evaluation result becomes “safe”. However, if a foreign object enters such a battery, the battery may become internally short-circuited at a given location, depending on the size, shape, hardness, etc., of the foreign object. That is, nail penetration test methods cannot accurately evaluate safety under possible internal short-circuit conditions in the market.
Also, with respect to crush test methods, it has been found from the analysis of short-circuit behavior in crush tests that a plurality of locations are short-circuited at one time or the short-circuit location varies among tests. It is therefore believed that crush test methods also cannot accurately evaluate safety under internal short-circuit conditions.